A critical question in stem cell biology is how stem cells escape cell division stop signals. We have shown the necessity of the microRNA (miRNA) pathway for proper control of germ line stem cell (GSC) division in Drosophila melanogaster. Analysis of GSCs mutant for dicer-1 (dcr-1), the dsRNAseIII essential for miRNA biogenesis, revealed a dramatic reduction in the rate of germline cyst production. These dcr-1 GSCs exhibit normal identity but are defective in cell cycle control. Based on cell cycle markers and genetic interactions, we conclude that dcr-1 GSCs are delayed in the CDK-inhibitor p21/p27/Dacapo-dependent G1 to S transition, suggesting that miRNAs are required for stem cells to bypass the normal G1/S checkpoint. Hence, the miRNA pathway might be part of a mechanism that makes stem cells insensitive to environmental signals that normally stop the cell cycle at the G1/S transition. We are now in the process of analyzing the microRNAs critical for stem cell division and identifying the region of Dacapo 3’UTR responsive to these microRNAs. Germ line stem cells reside in a microenvironment, niche where they undergo asymmetric division to produce the differentiating cells destined to develop into mature eggs. In the absence of injury, it has been thought that these stem cells persist for the life of the organism. We have shown that this is not the case: stem cells are replaced every three to four days throughout the life of the adult fly. Continuous replacement may provide a robust means of maintaining the stem cell population and contribute to its longevity.

Using a genetically tractable Drosophila model for studying muscular dystrophy, we have dissected the function of the Dystroglycan(DG)-Dystrophin(Dys) complex in muscle and in the brain. Genetic and RNAi based perturbation of DG and Dys causes both cell polarity and muscular dystrophy phenotypes: decreased mobility, shortened lifespan, age-dependent muscle degeneration and defective photoreceptor path-finding. In the latter case, we find that the DG-Dys complex is essential both in the photoreceptor neurons and the targeting glial cells, suggesting that both cell types are involved in ECM based regulation of axon pathfinding. Using a fluorescence polarization assay, we have shown that the DG-Dys interaction is remarkably well evolutionary conserved between flies and humans. Surprisingly structure-function studies of DG revealed that a truncation of the WW-binding motive, thought to interact with Dystrophin is not essential for DG function. However, one mutation to an alanine of a proline within a SH3-domain binding site abolishes DG function suggesting a critical interaction with a potential signaling molecule that might regulate the complex. Genetic and biochemical screens are in progress to reveal the critical interacting genes.